Engine

P03BA — Cylinder 4 Pressure Variation Low

Severity: medium Drive with care, to a shop

What P03BA means

The combustion monitoring system, using a cylinder pressure sensor or crankshaft speed analysis, tracks how consistent combustion is from one cycle to the next. P03BA means cylinder 4 shows less pressure variation than expected, so it is weak or barely contributing. This usually points to a misfire, poor fuel delivery or lost compression.

Symptoms

  • Check Engine light on with P03BA stored
  • Rough or unstable idle
  • Reduced power and sluggish acceleration
  • Engine vibration or a misfire feel
  • Possible higher fuel use and emissions

Possible causes

  1. Faulty fuel injector on cylinder 4
  2. Worn spark plug or failing ignition coil on cylinder 4
  3. Low compression in cylinder 4 (valves, rings, head gasket)
  4. Vacuum leak or uneven intake airflow
  5. Faulty cylinder pressure sensor or its circuit
  6. Damaged wiring or poor connector contact for cylinder 4

How to diagnose

  1. Read all codes and freeze frame data, and look for misfire codes such as P0304
  2. Inspect the cylinder 4 spark plug and coil; swap the coil with another cylinder and see if the fault follows
  3. Test the cylinder 4 injector: resistance, drive signal, operation and balance
  4. Check the intake for vacuum leaks, hoses and manifold gasket
  5. Inspect wiring and connectors for cylinder 4 and the pressure sensor, if fitted
  6. Perform a compression or cylinder leakdown test
  7. Compare live data between cylinders with a scan tool

Common fixes

  • Replace the spark plug and/or ignition coil on cylinder 4
  • Replace or clean the cylinder 4 fuel injector
  • Repair intake vacuum leaks
  • Repair mechanical engine problems causing low compression (valves, head gasket, rings)
  • Repair wiring or replace the cylinder pressure sensor

Questions and answers

What does P03BA mean?

The ECU detected that combustion pressure in cylinder 4 varies less than it should from cycle to cycle. It is most often caused by a misfire, an injector problem or lost compression.

Can I drive with P03BA?

Only briefly and gently, to get to a shop. Prolonged driving with a misfire can damage the catalytic converter and the engine.

What should I check first?

Start with the spark plug and ignition coil on cylinder 4, since they are the easiest and most common cause. Then check the injector, vacuum leaks and compression.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.